Mesostructured Silica Thin Films Research Articles

Sol–Gel‐Based Advanced Porous Silica Materials for Biomedical Applications

AbstractPorous silica‐based materials have burgeoning applications ranging from fillers and additives, to adsorbents, catalysts, and recently therapeutic agents and vaccines in nanomedicine. The preponderance of these materials is made by sol–gel processing wherein soluble silica precursors are reacted to form amorphous networks composed of siloxane bonds. The facile sol–gel approach allows for an unlimited variety of binary tertiary and more complex chemical compositions including organic ligands and networks resulting in so‐called organic–inorganic hybrid materials. Here, a brief review of the recent progress in sol–gel‐derived silica materials prepared as particles, thin films, biosilica/silica bioreplicas (of molecules, cells and organisms), and their related preparation, properties, and bioapplications is provided. First, it highlights the recent achievements of mesoporous silica nanoparticles in biomedical applications, including therapeutic agent delivery, multimodal imaging and theranostics, and bone tissue engineering and repair. Second, the research in evaporation‐induced self‐assembly (EISA)‐based mesostructured silica thin films and cell‐directed EISA in bio/nano interfaces for various bioapplications, such as bioactive coatings, biosensing, and living cell immobilization, has been reviewed. Third, the pioneering work in biomimetic silicification/immobilization of biomolecules and bio‐organisms and silica bioreplication of complex bio‐organisms is summarized. Finally, it is concluded with personal perspectives on the directions of future work on this field.

Vertically Aligned and Ordered One-Dimensional Mesoscale Polyaniline.

The growth of vertically aligned and ordered polyaniline nanofilaments is controlled by potentiostatic polymerization through hexagonally packed and oriented mesoporous silica films. In such small pore template (2 nm in diameter), quasi-single PANI chains are likely to be produced. From chronoamperometric experiments and using films of various thicknesses (100-200 nm) it is possible to evidence the electropolymerization transients, wherein each stage of polymerization (induction period, growth, and overgrowth of polyaniline on mesoporous silica films) is clearly identified. The advantageous effect of mesostructured silica thin films as hard templates for the generation of isolated polyaniline nanofilaments is demonstrated from enhancement of the reversibility between the conductive and the nonconductive states of polyaniline and the higher electroactive surface areas displayed for all mesoporous silica/PANI composites. The possibility to control and tailor the growth of conducting polymer nanofilaments offers numerous opportunities for applications in various fields including energy, sensors and biosensors, photovoltaics, nanophotonics, or nanoelectronics.

Strong Acid-Nonionic Surfactant Lyotropic Liquid-Crystalline Mesophases as Media for the Synthesis of Carbon Quantum Dots and Highly Proton Conducting Mesostructured Silica Thin Films and Monoliths.

Lyotropic liquid-crystalline (LLC) materials are important in designing porous materials, and acids are as important in chemical synthesis. Combining these two important concepts will be highly beneficial to chemistry and material science. In this work, we show that a strong acid can be used as a solvent for the assembly of nonionic surfactants into various mesophases. Sulfuric acid (SA), 10-lauryl ether (C12E10), and a small amount of water form bicontinuous cubic (V1), 2D-hexagonal (H1), and micelle cubic (I1) mesophases with increasing SA/C12E10 mole ratio. A mixture of SA and C12E10 is fluidic but transforms to a highly ordered LLC mesophase by absorbing ambient water. The LLC mesophase displays high proton conductivity (1.5 to 19.0 mS/cm at room temperature) that increases with an increasing SA content up to 11 SA/C12E10 mole ratio, where the absorbed water is constant with respect to the SA amount but gradually increases from a 2.3 to 4.3 H2O/C12E10 mole ratio with increasing SA/C12E10 from 2 to 11, respectively. The mixture of SA and C12E10 slowly undergoes carbonization to produce carbon quantum dots (c-dots). The carbonization process can be controlled by simply controlling the water content of the media, and it can be almost halted by leaving the samples under ambient conditions, where the mixture slowly absorbs water to form photoluminescent c-dot-embedded mesophases. Over time the c-dots grow in size and increase in number, and the photoluminescence frequency gradually shifts to a lower frequency. The SA/C12E10 mesophase can also be used as a template to produce highly proton conducting mesostructured silica films and monoliths, as high as 19.3 mS/cm under ambient conditions. Aging the silica samples enhances the conductivity that can be even larger than for the LLC mesophase with the same amount of SA. The presence of silica has a positive effect on the proton conductivity of SA/C12E10 systems.

In Situ FTIR Investigation of the Kinetics of Silica Polycondensation in Surfactant Templated, Mesostructured Thin Films

A detailed Fourier transform infrared spectroscopy (FTIR) study of the kinetics of polycondensation of surfactant-templated mesostructured silica thin films was carried out with the goal of understanding how to manipulate the synthesis and processing of these films to achieve a desired architecture. The evolution of silica condensation was followed both during the sol preparation process and in situ for a time period on the order of minutes to hours after initial film deposition. The kinetics were measured in the presence of three commonly used classes of surfactant templates: P123 [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer), Brij-56 (poly(ethylene oxide] [n ∼ 10] hexadecyl ether), and CTAB (cetyltrimethylammonium bromide). The induction period for polycondensation (an initial period during which the rate of change of silanol content is slow) was different in each case, with P123 giving the longest induction time followed by Brij-56 and finally CTAB. Humidity was f...

Growth by heat treatment of silver nanorods inside mesostructured silica thin films: Synthesis, colours of thin films, study of some experimental parameters and characterization

In this paper we report a soft route leading to spherical or rod nanoparticles confined in SBA-15 thin films exhibiting a hexagonal mesostructure. For that purpose, these transparent mesoporous silica thin films are elaborated in the presence of a non ionic triblock copolymer P123 as structure directing agent. After soaking in a silver salt solution, they are heat treated to grow silver structures. In function of several experimental parameters as the immersion time in the silver salt solution, the solvent of the silver salt solution or the heat treatment temperature, nanorods and/or nano spherical particles can be obtained. These coloured samples are characterized by X-ray diffraction (XRD), ultraviolet–visible (UV–vis) absorption and Fourier transformed infrared (FTIR) spectroscopies, atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The presence of the nanorods has been showed by using polarized UV–vis spectroscopy. The results show that most of silver nanoparticles grow inside the film thickness.

Monosteps on the Surfaces of Mesostructured Silica and Titania Thin Films

Steps are observed on the surfaces of mesostructured silica and titania thin films that are several layers thick. The heights of the steps are equal to the thickness of one mesostructural layer. The areas enclosed by the steps are approximately circular.

Structural Characteristics of Oriented Mesostructured Silica Thin Films

Mesoporous thin films synthesized via an electrochemical strategy (ref 1) generally show granular domains, each of which is composed of hexagonally packed one-dimensional channels oriented uniquely perpendicular to the film surface. Grain boundaries either parallel or normal to the channel direction might affect the properties and subsequent application of the film. In this study, the structural details of oriented mesostructured silica thin films have been examined by transmission electron microscope. The pore structures are characterized using the traditional crystallographic concepts but show different structural properties from that of polycrystalline materials. The boundary structures vary much depending on the residual internal stress and the orientation relationship between the bounded grains. A variety of structural features, typically near the large-angle tilt boundaries, have been observed including coincidence site lattices, lattice distortion, lattice displacement, and dislocations. According to the present structural analysis, microstructure evolution and potential applications have been discussed with respect to the oriented mesoporous films.

Incorporation of Gold Nanorods and Their Enhancement of Fluorescence in Mesostructured Silica Thin Films

Gold nanorods with longitudinal surface plasmon resonance wavelengths ranging from 544 to 840 nm have been coated with silica at varying thicknesses. Silica-coated Au nanorods are vitreophilic, and mesostructured composite thin films have thereafter been prepared by dispersing silica-coated Au nanorods in the silica precursor solution. Au nanorods are distributed relatively uniformly in the resultant films and their incorporation amounts can be readily varied. Mesostructured composite films containing both Au nanorods and fluorophores have been further fabricated. Each Au nanorod in these films can be treated as being embedded in a uniform array of fluorophores. Dark-field and fluorescence imaging have been carried out in the same area of the films to correlate the scattering from single nanorods with the fluorescence from the surrounding fluorophores. An exact correspondence between the bright spots on the dark-field image and those on the fluorescence image indicates that the fluorescence of the fluorophores is enhanced by nearby Au nanorods. The fluorescence enhancement is found to be dependent on different fluorophores and gets larger as the thickness of the precoated silica layer on Au nanorods becomes thinner.

Facile and adaptable synthesis method of mesostructured silica thin films

By employing high speed spin coating for the film formation and high temperature (80 °C) drying and aging, we developed a synthesis procedure for mesostructured silica thin films that can reproducibly produce up to eight different pore morphologies, including a rhombohedral (Rm), a centered rectangular (c2mm), two wormlike, two curved lamellar, and two lamellar structures. The mesostructured films were characterized by low angle X-ray diffraction and transmission electron microscopy, and the rhombohedral and centered rectangular by N2 adsorption in addition. The two measures we used in our procedure are believed to effectively quench the hydrolysis/condensation reactions of silica species after the coating sol–gel solution is cast into thin films, so that the mesostructures in the films are formed through the liquid-crystal mechanism. In this way, the final mesostructure is determined by the size and size distribution of the silica species, as probed by 29Si-NMR spectroscopy, which are determined by the water and/or HCl contents in the coating solutions. We have been able to synthesize mesostructured films with large areas (up to 4 inch in diameter) and on various substrates (ITO, FTO, Si wafer, Pt, Au, and glass) without inducing any changes of the mesostructures. The mesostructured films grown on conductive substrates could serve as templates to grow electrochemically nanostructures of various materials that replicate the pore structures. The metal replicas provide deeper insights into the structures of the mesostructured films by revealing some features such as grain boundaries that are not obvious from the analyses of the mesoporous silica films only.

Light-activated functional mesostructured silica

Mesostructured silica thin films and particles provide highly versatile supports or frameworks for functional materials where a desired function (such as energy transfer, electron transfer, or molecular machines) is induced by molecules deliberately placed in specific regions of the structure. The relatively gentle templated sol–gel synthesis methods allow a wide variety of molecules to be used, and the optical transparency of the framework is very suitable for studies of light-induced functionality. In this paper, three types of functionality are used to obtain fundamental understanding of the materials themselves and to develop active materials that can trap and release molecules from the pores upon command. Photo-induced energy transfer is used to verify that molecules can be placed in specific spatially separated regions of the framework; fluorescence resonance energy transfer is used as a molecular ruler to measure quantitatively the distance between pairs of molecules. Secondly, photo-induced electron transfer is used to obtain fundamental information about the electrical insulating properties of the framework. Finally, two types of molecular machines, a light-driven impeller and a light activated nanovalve, are described. Both machines contain moving parts attached to solid supports and do useful work. The valves trap and release molecules from the mesopores, and the impellers expel molecules from the pores. Applications of the materials to drug delivery and the release of drug molecules inside living cells is described.

Charged molecular effect on the degree of ordering of mesostructured silica thin film templated by mixed ionic micelles

We have synthesized highly oriented mesostructured silica thin films templated with nonionic and cationic mixed surfactant micelles by a solvent evaporation-induced self-assembly process. The mesostructured silica thin films showed constant body-centered cubic lattice structures over a wide range of mixing mole ratios of the cationic surfactant in the nonionic surfactant-rich phase. The degree of ordering of the mixed micellar arrays in the mesostructured silica thin films was enhanced by tuning the mixing mole ratio of the cationic surfactants to nonionic surfactant.

Rare earth doped mesoporous hybrid thin films with tunable optical responses

Optical quality mesostructured silica thin films functionalized with three large hydrophobic organosilylated quinizarin derivatives are prepared via evaporation induced self-assembly (EISA). Incorporation of Eu3+ is performed by post-reacting the functionalized layers with several europium precursors. Unambiguous location of the quinizarin moieties inside the porosity and their accessibility to Eu3+ ions are demonstrated using XRD, SAXS and fluorescence measurements. Complexation of Eu3+ reduces the fluorescence of quinizarin; for some europium precursors an energy transfer between the grafted organic dye and the lanthanide is clearly observed. The luminescence intensity of Eu3+ can be tuned by varying the nature of the rare earth precursor, the mesophase and the chelate itself. The resulting optical responses differ with respect to concentration, lifetime and local environment of Eu3+ inside the thin films. Additionally, efficient energy transfer from Tb3+ to Eu3+ and electronic coupling probed by EPR between Cu2+ cations trapped in such mesoporous hybrid films give strong evidence of the presence of several metal ions per pore.

Structural study of highly ordered mesoporous silica thin films and replicated Pt nanowires by high-resolution scanning electron microscopy (HRSEM)

We have investigated the structures of mesostructured (before calcination) and mesoporous (after calcination) silica thin films and replicated Pt nanowires by high-resolution scanning electron microscopy (HRSEM). Highly ordered lamellar, two-dimensional (2D) hexagonal, and three-dimensional (3D) hexagonal mesostructured/mesoporous silica thin films were synthesized by adjusting the concentration of triblock copolymer (EO20PO70EO20; P123) used as a structure-directing agent. Direct HRSEM observation of these mesostructured/mesoporous silica thin films demonstrated that they have inherent external structures on the top surfaces: (1) the lamellar mesostructured silica film exhibits a smooth plane; (2) the 2D hexagonal mesoporous silica film exhibits curved stripes; (3) the 3D hexagonal mesoporous silica film exhibits 2D porous arrangements (e.g. 6-fold symmetry and pseudo 4-fold symmetry). The removal of P123 by calcination had no effect on the surface structure but caused an anisotropic contraction, as found from cross-sectional images. Methods for the formation of the 2D hexagonal mesoporous film affected the orientation of mesochannels. Pt replicates, electrodeposited in the mesochannels of the 2D hexagonal mesoporous silica films, exhibited new structures, i.e., S-shaped, Y-shaped, and swirling-shaped nanowires with ellipsoidal cross section. Direct HRSEM observation of the mesostructured/mesoporous silica thin films on the top surface and cross section provides detailed information on the external and internal structures, which is significant and useful for various applications of these thin films.

Mechanical properties of mesoporous silica thin films: Effect of the surfactant removal processes

Mechanical properties including hardness and Young's modulus of ordered mesostructured silica thin films are characterized by nanoindentation measurements. PE6800 and CTAB are used as templating agents. Emphasis is given on the effect of the template removal by thermal treatment, solvent extraction or UV-Ozone irradiation. The mechanical properties of the film are intimately correlated to the microstructure of the silica network and the film porosity as studied by IR, X-ray diffraction and N 2 adsorption. Particularly, the position and shape of the longitudinal optic mode of the Si–O–Si asymmetric stretch are correlated to the mechanical response as a function of the treatments. A contrasted behaviour is observed between PE6800 templated films which present low initial condensation state and high network contraction after post-synthesis treatments, and CTAB templated films which exhibit higher initial silica condensation degree but lower compressibility after template removal. These distinct behaviours are correlated to the strengthening of the silica structure and the decrease of the film porosity during template removal, which evolve differently according to the nature of the template and the template removal processing.

Template Cross-Linking Effects on Morphologies of Swellable Block Copolymer and Mesostructured Silica Thin Films

Mesostructured silica films were formed from cross-linked poly(d8-styrene-block-2-vinylpyridine) (dPS-b-P2VP) diblock copolymer template films by swelling with silica precursor solutions. The mesoscopic and macroscopic morphologies of the resulting materials depended strongly on the extent of P2VP cross-linking. Without cross-linking, extensive rearrangement of the initial 2D hexagonal arrangement of dPS cylinders took place during swelling, leading to wormlike mesostructures. At low extents of template cross-linking, osmotic stresses led to a surface-wrinkling instability with a morphology directed by the anisotropic mechanical properties of the swelled block copolymer mesostructure. At higher degrees of P2VP cross-linking, wrinkling was suppressed, and the composite silica/polymer mesostructures corresponded to essentially affine expansions of the initial template structures normal to the film plane. At low extents of P2VP cross-linking, the mesostructures of the hybrid block copolymer/silica films were preserved during oxidation and removal of the block copolymer template species, leading to stable mesoporous silica films. At high extents of P2VP cross-linking, however, insufficient amounts of silica were incorporated to form stable porous inorganic materials, leading to mesostructural collapse. An intermediate level of cross-linking allowed the preparation of stable mesoporous silica films with structures that closely resembled those of the initial template films.

An in situ study of mesostructured CTAB-silica film formation using infrared ellipsometry: evolution of water content

The role of the relative humidity in the formation of mesostructured silica thin films prepared by dip-coating in controlled environment, combining cethyltrimethylammonium bromide as surfactant and tetraethoxysilane as inorganic precursor, is studied for the first time by in situ infrared ellipsometry in the 600–5000 cm −1 spectral range. The change in thickness and optical constants during drying was followed regularly at times between 2 and 60 min after deposition. Because the water content is likely to play a major role in the film structuration, its value has been calculated from the analysis of the infrared optical constants. The decrease of the thickness during drying is attributed to the release of water.

Nano-architectural silica thin films with two-dimensionally connected cagelike pores synthesized from vapor phase.

Novel mesostructured silica thin films were prepared on a Si substrate by a vapor-phase synthesis. Vapor of tetraethoxysilane (TEOS) was infiltrated into a surfactant film consisting of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer. Nanophase transition from a lamellar structure to a two-dimensional cage structure of a silica-surfactant nanocomposite was found under vapor infiltration. The rearrangement into the cage structure implies high mobility of the silica-surfactant composites in solid phase. The silica thin films have two-dimensionally connected cagelike mesopores and are isotropic parallel to the film surface. The structure of pores of the films is advantageous for next-generation low-k films. The mesoporous structure has a large lattice parameter d of approximately 102 A, silica layer thickness of approximately 58 A, pillar diameter in the middle of approximately 60 A, pore size of approximately 72 A, BET surface area of approximately 729 m(2)/g, and pore volume of approximately 1.19 cm(3)/g. The films synthesized by the vapor infiltration show a lower concentration of residual Si-OH groups compared to the films prepared by a conventional sol-gel method. The films show high thermal stability up to 900 degrees C and high hydrothermal stability. This method is a simpler process than conventional sol-gel techniques and attractive for mass production of a variety of organic-inorganic composite materials and inorganic porous films.

Mesostructured Silica Thin Films with Spherical Voids Organized in a BCC Array Achieved

Mesostructured Silica Thin Films with Spherical Voids Organized in a BCC Array Achieved

Energy Transfer in Dye-Doped Mesostructured Composites

Energy transfer between the organic laser dyes coumarin 485 and pyrromethene 567 was investigated in mesostructured silica/block copolymer thin films. The 2-D hexagonal mesostructured silica thin films were made using acidic sol−gel synthesis with poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide), (EO20PO70EO20) as the structure-directing agent. The dyes were doped into the film synthesis solutions so that they underwent co-assembly with the silica and block copolymer. At a very low concentration of acceptor, one acceptor molecule for every 20 donor molecules, the donor's emission was drastically quenched, suggesting efficient energy transfer. Investigations into amplified spontaneous emission revealed that the donor/acceptor system resulted in full gain narrowing at significantly lower concentrations than systems without a donor.

Direct Synthesis and Characterization of High Titanium-Loading Hexagonal Mesostructured Silica Thin Films

High titanium-loading hexagonal mesostructured silica thin films (Ti-HMSTF) have been successfully synthesized by carefully controlling two factors. One is the hydrolysis and condensation reaction of titanium alkoxide and the other is the aging condition of as-made Ti/Si mixed thin films. The former was controlled by adding acetylacetone (AcAc) as a Ti chelating agent. Regarding the latter, aging under a hydrothermal water vapor ambient environment was found to be effective in synthesizing Ti-HMSTFs with well-defined mesostructures. The maximum molar ratio of Ti/Si in the Ti-HMSTF materials attained a value of 0.3 (referred to as Ti-HMSTF-0.3) for the as-made films. These materials were subjected to a specific hydrothermal aging process, which was prepared from a precursor solution containing AcAc with the molar ratio AcAc/Ti=1. Small angle X-ray diffractometry (SA-XRD) and transmission electron microscopy (TEM) demonstrated that Ti-HMSTF-0.3 had a highly ordered 2-dimensional hexagonal mesostructure. This 2D hexagonal mesostructure was thermally stable even after the removal of the triblock copolymer template by calcination at 450°C for 4 h. Moreover, small amounts of TiO2 anatase nanocrystals with a size of about 3 nm were formed in the calcined Ti-HMSTF-0.3. O(1s) X-ray photoelectron spectroscopy (XPS) analysis indicated that the incorporation of titanium into the HMSTF was through the Si–O–Ti bonds. The Ti(2p) XPS showed that the binding energy of the titanium in HMSTF decreased with increasing Ti loading.